The similarity between the Arthur of legend and Lucius Artorius Castus is that they shared the same name, both were military leaders in Britain, and both campaigned in Gaul (or maybe Lucius actually campaigned in Armenia?)
Can this be a coincidence?
Of course it can, and therefore the connection is usually considered to be yet another theory that has not been proved.
See http://www.christophergwinn.com/celticstudies/lac/lac.html for a very complete discussion of ALL the evidence. It will probably put most readers to sleep.
This also contains links to articles by Linda Malcor. Christopher Gwinn claims “Linda Malcor’s biography of LAC is error-laden and relies too much on wild speculation to be used as a credible source.” I agree.
A book by C. Scott Littleton and Linda A. Malcor named “From Scythia to Camelot” gives some of Malcor’s theories. A favorable view of the book is found at http://www.mun.ca/mst/heroicage/issues/2/ha2tf.htm . However, in contrast, I found the book to be full of wild suppositions and obvious errors. Mostly bogus research.
Malcor claims to be following in the path laid out by George Dumézil, but many, including myself who find most of Dumézil’s writing to be convincing find Malcor to be anything but convincing. She appears to be more of a crank with fixed ideas into which anything she finds are fitted, unable to think outside the box she has built for herself.
Lucius Artorius Catus is just part of Malcor’s Sarmatian hypothesis. Her arguments are circular. Motifs found in Arthurian legends (or indeed, any legend), that can be connected with Sarmatians become, for her, proof that these motifs were originally Sarmatian motifs, and therefore Lancelot and Arthur were originally the same legendary person (no evidence provided).
Linda Malcor haunted the Arthurnet discussion group until the inability of herself or her followers to even understand why some were skeptical and an equal inability to at least shut up led to any discussion of the Sarmatian hypothesis being banned. See http://www.clas.ufl.edu/users/jshoaf/arthurmalcor.html .
That “arthur” was a word for “leader” is just an hypothesis. No evidence, other than the suggestion that if “arthur” were a word for “leader”, then a particular theory might be entertained. Note that the name “Artorius” was not a word for “leader’'.